Bearing steel is used in a rolling bearing such as a “ball bearing” or a “roller bearing” which is used in various industrial machines or vehicles. In addition, in recent years, for example, the bearing steel has also been used in a bearing for a disk drive or the like used in a hard disk device which is a magnetic recording medium. In addition, the bearing steel is also used in a bearing for an electronic device, a household electric appliance, a measuring instrument, a medical instrument, or the like.
It is required for the bearing steel used in the bearings to be excellent in rolling fatigue properties. When the bearing steel includes coarse inclusions and a large number of inclusions, fatigue life is negatively influenced. Accordingly, it is desirable that the inclusions are as fine as possible and a number of the inclusions is as small as possible in order to improve the fatigue properties.
Oxides such as alumina (Al2O3), sulfides such as manganese sulfides (MnS), nitrides such as titanium nitrides (TiN), or the like is known as harmful inclusions included in the bearing steel.
A large amount of dissolved oxygen is included in molten steel made in a converter or a vacuum processing chamber. Alumina inclusions are formed by combining the dissolved oxygen and Al having high affinity for the oxygen.
In addition, a ladle or the like used in steel-making process is often constructed by using an alumina-based refractory. Accordingly, even when the molten steel is deoxidized by using Si or Mn instead of Al, Al is dissolved in the molten steel due to reaction between the molten steel and the above refractory, the dissolved Al is reoxidized, and thereby, the alumina is formed in the molten steel. The alumina inclusions are hard and form a coarse alumina cluster by being agglomerated and combined, which causes a deterioration of the fatigue properties.
For decreasing and removing the alumina inclusions, the deoxidization products are mainly decreased by applying a secondary refining apparatus such as a RH (Ruhrstahl-Hausen) vacuum degassing apparatus or a powder injection apparatus. In addition, reoxidization is suppressed by shielding air or reforming slag, the oxides are prevented from mixing by slag cut, and a combination thereof is conducted for a decrease in the inclusions.
For example, a following producing method is known. In the producing method, when an Al-killed steel containing 0.005 mass % or more of Al is produced, the alumina in the formed inclusions is controlled to be in a range of 30 mass % to 85 mass % by adding alloy consisting of Al and at least two selected from Ca, Mg, and REM (Rare Earth Metal) into molten steel.
Patent Document 1 discloses technology of controlling the formed inclusions to be complex inclusions whose melting point is low by adding at least two selected from REM, Mg, and Ca to molten steel in order prevent the formation of the alumina cluster. Although the technology may be effective in preventing sliver defects, it is difficult to refine the size of the harmful inclusions to the required level for the bearing steel. This is because the inclusions are further coarsened by being agglomerated and combined when the inclusions are controlled to be the inclusions whose melting point is low.
In a case where REM is added, since REM makes the inclusions spheroidal, an effect in improving the fatigue properties is obtained. Although REM is added as necessary in order to control morphology of the inclusions as described above, addition of more than 0.010 mass % of REM results in an increase in the inclusions and a decrease in the fatigue life. For example, Patent Document 2 discloses the necessity to control REM content to be 0.010 mass % or less. However, Patent Document 2 does not disclose the mechanism or a composition and an existence condition of the inclusions.
The shape of sulfides such as MnS is elongated by plastic deformation such as forging. Thus, when repeated stress is applied, the sulfides accumulate the fatigue and act as a fracture origin, and thereby, the fatigue properties deteriorate. Accordingly, it is necessary to control the sulfides in order to improve the fatigue properties.
As a method of suppressing the formation of the sulfides, a method of desulfurization using Ca is known. However, Al—Ca—O complex oxides which are formed by adding Ca have problems such that the shape thereof tends to be elongated by the plastic deformation and that the complex oxides accumulate the fatigue and act as the fracture origin when the repeated stress is applied. In addition, the utilization of Ca is not effective in TiN.
Since the nitrides such as TiN are extremely hard and are precipitated with a sharp shape, the nitrides accumulate the fatigue and act as the fracture origin, and thereby, the fatigue properties deteriorate. For example, Patent Document 3 discloses that, when Ti is added exceeding 0.001 mass %, the fatigue properties deteriorate.
In order to suppress the formation of TiN, it is important to control Ti content to be 0.001 mass % or less. However, since Ti is also included in molten iron or slag, it is difficult to stably reduce the content. Thus, it is necessary to effectively remove Ti and N in the molten steel, but the production cost of the steel unpreferably increases.
As described above, the technology of controlling the harmful inclusions such as Al2O3, Al—Ca—O complex oxides, MnS, and TiN in order to improve the fatigue properties required as the bearing steel is not found at present.